The need for electrically conductive organic polymeric structures has increased. One method to achieve such structures is the formation of a composite between the organic polymer and a metal. Flexible conductors of this type are useful for electromagnetic wave shielding materials and other applications. The reduction in size of electronic devices requires greater flexibility, durability and softness for conducting materials, and such reduction is most easily achieved by the use of a fabric from a metal-coated organic polymer fiber. The manner in which the conducting fibers have been prepared has varied depending upon the desired metals that have been placed on the organic polymer. Silver and copper are the two primary metals deposited on organic polymers for these applications because these metals have very high conductivities. Unfortunately, these two metals easily undergo corrosion and are inherently soft, lacking the durability required for many applications.
The deposition of silver on organic polymers is well known. This is described in United States Patent Application Publication 2004/0173056. Likewise, the deposition of copper on organic polymers is well known and is described in U.S. Pat. No. 4,228,213.
Nickel is frequently deposited on a metal to enhance the surface properties of the metal. Usually this is carried out by an electroless plating process. Although an electrodeposition process can produce a nickel-plated structure, it requires a conductive substrate and gives a different coating than an electroless plated structure. The electroless plated structure typically displays less pure nickel but the coating is typically thicker and more even. The electroless plated nickel is generally superior in corrosion resistance.
The electroless plating process is often carried out by the addition of a reducing agent to a solution containing a metal salt. For the deposition of nickel, common reducing agents include sodium hypophosphite, sodium borohydride, dimethylamine borane, and hydrazine. Depending upon the reducing agent that is used, the metal displays some content of phosphorous, boron, or nitrogen. The nickel deposits are generally characterized as high phosphorous, low phosphorous, high boron, and so forth. When sodium hypophosphite is used as the reducing agent, phosphorous can range from about one percent to about 15 percent of the nickel coating. The properties of the coating depend upon the amount of the non-nickel content. Properties that can vary include conductive, magnetic and corrosion resistance properties.
The most commonly used reducing agent for electroless nickel deposition is sodium hypophosphite. The process can be described by the following equation:Ni+2+H2PO2−+H2O→Ni0+H2PO3−+2H+This reaction competes with the following reaction:H2PO2−+H2O→H2PO3−+H2↑Both of these reactions involve the adsorption of atomic hydrogen on a catalytically active surface. The adsorbed hydrogen either combines to form hydrogen gas or transfers an electron to reduce the nickel ion to nickel metal. The adsorbed hydrogen is believed to be responsible for the reduction of hypophosphite to phosphorous, and the phosphorous is incorporated into the nickel coating.
The electroless deposition technique requires the formation of the catalytically active surface prior to the autocatalytic reduction of nickel (II) to nickel metal on the surface. The nature of the catalyst added to generate the catalytically active substrate surface is dependent on the substrate, and for noble metals and non-metals, the common catalyst is a palladium species. A particularly effective system uses stannous chloride and palladium chloride to form the catalytically active surface. Typically, a colloid is formed from a reaction of palladium chloride and stannous chloride in the presence of excess hydrochloric acid to treat the surface for electroless plating of nickel.
In addition to the catalyst to form the active surface, a typical deposition bath requires a complexing agent, a pH regulator, an accelerator, a stabilizer, a buffer, a wetting agent and a reducing agent to achieve a desired metal coating. This complex mixture unfortunately results in a waste stream that is complicated to process. A typical electroless nickel bath is spent after three or four turnovers at which time it is considered waste. This spent bath typically contains nickel at a concentration of more than 5,000 mg per liter, unreacted reducing agent, oxidized reducing agent, and all of the other components previously mentioned. The spent bath is usually treated with hydrated lime to precipitate nickel salts and the remainder of the sludge, which still has significant quantities of nickel, is frequently sent to a landfill with potential environmental risks and, in the United States, an economic risk to the generator of the waste stream.
Numerous studies directed toward the reduction and treatment of waste from electroless nickel plating processes have been carried out and are in progress. The direction of these studies include alternate plating chemistries, plate out of residual nickel, ion exchange and electrodialysis.
A need for a corrosion resistant highly conductive metal-coated plastic substrate remains. More specifically, a need exists for a composite including the flexibility and strength of a polymeric substrate and a highly conductive metal that is resistant to corrosion. Furthermore, a need exists for an electroless nickel process that permits many turnovers of a bath and leaves little or no nickel in the spent bath, thereby reducing expenses associated with environmental cleanup.